Dobot magician air pump not working both ways


In my class I have 3 dobot magicians. 1 air pump works fine. The other 2 do not. We use the same arms, same wires and have retried everything multiple times. Opened the box and see no obvious damage, disconnections, or leaks. When we program the gripper, the gripper will open no problem, but will not close when programmed to do so. Again the same gripper will work perfectly when the 1 box (air pump) that we have working is connected but not for the other 2. Please advise.